Understanding the mechanisms of aromatase inhibitor resistance
William R Miller, Alexey A Larionov
https://pmc.ncbi.nlm.nih.gov/articles/PMC3496110/
According to article cited above, there are at least 6 different kinds of resistance and different types of breast cancer resist differently. There is research into resistance--though I'm sure there could (should?) be more. As scientific articles go, the one above is relatively easy to understand--and worth a read IMHO.

PS My request is that when someone says they've done their research, that they post at least one link to the research study they have in mind.

PPS My request: If you've decided not to take AIs, please give a short account of which one(s) you've tried. If you haven't tried any, please say so.